The present invention relates to a priming composition for a base of cement mortar or concrete or, more particularly, to a priming composition used for improving the adhesion between the surface of a base cast with a cement mortar or concrete and a finishing overcoat layer formed with a cement paste or cement mortar as well as for increasing the water-proofing effect of the overcoat layer.
In the architectural techniques, it is very common that the surface of a concrete-made building or the like is finished by providing a finishing overcoat layer of a cement paste or cement mortar on the more or less rugged surface of a base body made of concrete not only in the case where the concrete body is built by in-place casting but also in the case where the body is constructed with precast concrete members. When the finishing overcoat is directly provided on the surface of the concrete base, a problem is sometimes unavoidable that the adhesion between the surface of the base and the overcoat layer is insufficient and the overcoat layer eventually exfoliates and comes off in the long run. This problem is more serious when the surface of the base is relatively smooth or the overcoating is worked on the base surface as dried. In addition, the finishing overcoat layer is susceptible to water permeation and waterproofing effect can hardly be expected, especially, when fissures are formed in the overcoat layer.
As a remedy for the above mentioned defects in the finishing overcoat with a cement mortar, there have been proposed and widely practiced priming methods by applying a priming composition to the surface of the concrete base or by admixing a modifying agent with the cement mortar for the finishing overcoat. These methods are effective to their own extents but cannot be free from several problems on the other hand. Conventional priming compositions include aqueous emulsions of polyvinyl acetate, copolymers of vinyl acetate and ethylene, copolymers of acrylic acid esters and styrene and the like as well as aqueous solutions of polyvinyl alcohol. Modifying agents to be admixed in the cement mortar for overcoat include aqueous emulsions of a copolymer of vinyl acetate and ethylene, latices of synthetic rubbers and the like.
The problems or drawbacks inevitably involved in the above mentioned methods are as follows. For example, the latter method of admixing a modifying agent into the cement mortar for finishing overcoat can exhibit satisfactory effectiveness only by the incorporation of a considerably large amount of the relatively expensive resinous or rubbery constituent in the cement mortar. Further, the aqueous emulsion used as a priming composition in the former method sometimes suffers from instability in the coating and drying steps on the base surface so that no uniform coating film having sufficient priming effect is obtained unless an excessively large amount of the emulsion is applied to an extent of 1.5 to 3 times larger than the amount expectedly sufficient when an evenly spread coating film is formed on the base surface. In addition, the waterproofing effect obtained with such a coating film formed with the emulsion is not always sufficiently high as is expected. The polyvinyl alcohol, which is used as an aqueous solution, is almost ineffective in waterproofing not only due to the hydrophilic nature of the polymer but also due to the low consistency of the polymer swollen with water. When an improved waterproofing effect is desired, a waterproof agent may be admixed with the aqueous solution of the polyvinyl alcohol. A serious problem in this case is that the priming effect of the polyvinyl alcohol is greatly reduced because waterproof agents are generally hydrophobic.